1

FundingFunding•National Science Foundation•R. A. Welch Foundation•Hungarian Science Foundation•Eötvös Fellowship•Bolyai János Fellowship•Széchenyi Professor Fellowship

Laszlo Turi Adam Madarasz(Eotvos Loring U., Budapest)

Wen-Shyan Sheu(Fu-Jen University, Taipei)

Daniel Borgis(Universite d’Evry / ENS Paris)

Excess Electrons in Water: Clusters, Interfaces, and the

Bulk

2

Water Cluster Anions: distinct “isomers”

Systematic variations

What are the characteristic properties which distinguish the different classes? Common sets of structural motifs?

Backing pressure/thermodynamic conditions. Non-equilibrium?

J. R. R. Verlet, A. E. Bragg, A. Kammrath, O. Cheshnovsky, and D. M. Neumark, Science, 307, 93 (2005).

3

Anionic clusters and hydrated electrons:localization mode/”binding motif” and

structure

↔

↔

“infinite” cluster

clusters

4

The Toolkit for Mixed Quantum-Classical MD Simulations

Components:

N classical water molecules (SPC model + internal flexibility)

the excess electron (wave function represented on dual [k,r] grid)

the electron-molecule interaction (pseudopotential*)

the force acting on the molecular nuclei:

= classical force (from the solvent) + quantum force (from the solute)

= FH2O + FQ

A sampling scheme: (adiabatic) time evolution of the system:

...)())(;())(;(ˆ)(12000

ttRQEtRQHtQOHnucl

RFFFR

{ quantum mechanical e- + classical solvent molecules }

* Turi, L.; Gaigeot, M.-P.; Levy, N.; Borgis, D.; J. Chem. Phys., 2001, 114, 7805. Turi, L.; Borgis, D. J. Chem. Phys., 2002, 117, 6186.

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Applicability of the Pseudopotential

Bulk:

VDE for n=12 clusters

MP2/6-31(1+3+)G*

vs.

the pseudopotential

0 100 200 300 4000

100

200

300

400

500

600

VD

EM

P2/

me

V

VDEpseudo

/meV

Turi, L.; Madarász, Á.; Rossky, P. J.; JCP 125, 014308 (2006).

E0 = -3.12 eV

Es-p,max = 1.92 eV (vs. 1.72)

RG = <r2>1/2 = 2.4 A

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Cluster Simulations: Surface states vs. internal states

L. Turi, W.-S. Sheu, P. J. Rossky, Science 309, 914 (2005), ibid. 310, 1719 (2005).

n = 20, 30, 45, 66, 104, 200 + 500, 1000

nominal T = 100K, 200K, 300K

(s p; n = 45. T = 200K)

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E0,1

gap

expt. (M. Johnson + coworkers)

Average surface state energetic behavior vs. interior states and vs. expt.

old lines, new points: n = 200, 500, 1000(surface and internal at 200K)

(expt)

(expt)

300K bulk

- spectral gap

E0

internal

internal

n -1/3

0 0.1 0.30.2

300K bulk

n -1/3

~35D

0.20

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Electron radius and kinetic energy

From: David M. Bartels - J. Chem. Phys. 115, 4404 (2001).

0 50 100 150 2000.0

0.5

1.0

1.5

2.0

2.5

Ekin

etic/e

V

n

0 50 100 150 200

1

2

3

4

5

Rg

,ele

ctro

n/Å

n

Simulations:

surface

surface

internal

internal

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Hydrated electrons at water/vacuum interfaces:

the infinite cluster limit

Cases: Ambient water surface (300 K) Supercooled water surface (200 K) Hexagonal ice surface (200 K) Amorphous solid (quenched) water surface (100 K)

Starting point: charge-neutral equilibrium surfaces

Dynamic simulations of surface accommodation

and final states

Localization analysis

Á. Madarász, P. J. Rossky, L. Turi, JCP 126, 234707 (2007).

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Interior and surface hydrated electrons at liquid water/vacuum interfaces

(meta)stable surface states at 200 K

vs. spontaneous internal states at 300 K

0 2000 4000 6000 8000 10000

-8

-6

-4

-2

0

2

4

6

8

(zco

m,e -

zG

ibbs

) / Å

t / fs

z(t)

10 ps

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Surface vs. Internal states

Internal state – bulk hydrated electron

Surface state – supercooled water interface

300 K Simulation temperature 200 K

2.4 Å Radius of the electron 2.7 Å

-3.1 eV Ground state energy -2.6 eV

1.9 eV Spectral maximum 1.5 eV

16 Coordination number (<5 Å) 10

12

partly reorganized -OH

Bulk Supercooled water interface

Amorphous solid water interface

ice Ih interface

Temperature 300 K 200 K 100 K 200 K

Electronradius

2.4 Å 2.7 Å 3.0 Å 2.6 Å

Ground state energy

-3.1 eV -2.6 eV -1.6 eV -2.7 eV

Spectral maximum

1.9 eV 1.5 eV ~1 eV 1.6 eV

Alternative surface states

partly reorganized from dangling -OH

fully reorganized -OH

restricted reorganization‘otherwise occupied’ -OH

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(Credit: Mark Johnson)

1

23

4 1 AA 2 AD 3 DD 4 AD ice AADD

D A

A

D

Donor-Acceptor characterization of water molecules

strong electron binding

Concept: N. I. Hammer, J.-W. Shin, J. M. Headrick, E. G. Diken, J. R. Roscioli, G. H. Weddle, and M. A. Johnson, Science, 306, 675 (2004).

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Hydrated electrons at solid water interfaces

1 2 3 4 5 60.0

0.5

1.0

1.5

2.0

2.5

3.0

0.0

0.5

1.0

1.5

2.0

2.5

3.0

fre

qu

en

cy c

ou

nt

r / Å

Structure II, T=100 K

Structure I, T=200 K

H-bonding structure analysis:

AA (solid) and AAD (dashed)

Ice Ih, 200K

ASW, 100K

AAD

AAD

AA

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Equilibrium and non-equilibrium preparation of cluster anions

quenched clusters (QC)

Prepare warm (ambient) neutral equilibrium structures

→ quench them gradually to a sequence of lower T’s Cluster surface site analysis

metastable clusters (MC)

Alternative preparation protocol: assemble the neutral clusters at very low T → warm them up gradually to the desired higher T.

metastable clusters have never “seen” annealing temperatures

Add the electron and relax (for ~ 200 ps).